1. Field of the Invention
The present invention relates to an integrated superconducting band-pass filter apparatus employing nonradiative dielectric waveguides (hereinafter referred to as "NRD waveguides").
2. Description of the Related Art
The following arrangement is disclosed in Japanese Unexamined Patent Publication No. 3-270401. An NRD waveguide (hereinafter referred to as a "first conventional example") is formed with upper and lower portions of a dielectric waveguide shaped, for example, in a quadrangular prism, which are interposed and held by a pair of flat metal plates. The vertical height of the dielectric member taken at right angles to the direction of its length is a half-wavelength or less, a brim is extended from one side to the other at the upper and lower end portions in order to form an H-shaped cross-section, and a metallic film is formed in close contact with the outer surfaces of both upper and lower ends of the dielectric member including the brim portion, thus forming a dielectric integrated NRD waveguide. Such a dielectric integrated NRD waveguide has a feature that even if vibration and/or impact are received, the metal section and the dielectric member are not separated from each other, and stable electrical characteristics can be obtained.
There has been proposed a dielectric-loaded waveguide filter or a waveguide-coupled NRD waveguide in which dielectric resonators at the initial and final stages are directly coupled to the waveguide. In the arrangement of such filters, there is a problem in that it is difficult to adjust the external Q and the resonance frequency independently from each other. In order to solve this problem, Japanese Unexamined Patent Publication No. 63-59001 proposes a waveguide-coupled NRD guide filter (hereinafter referred to as a "second conventional example") of a type in which an NRD guide resonator and a waveguide are directly coupled, wherein a buffer dielectric section is disposed in the connection portion of the NRD guide resonator and the waveguide, posterior to a resonator-forming dielectric section of the NRD guide resonator.
An NRD waveguide is formed by using low dielectric-constant materials as materials for dielectric waveguides of an NRD waveguide for use in the first and second conventional examples. If an NRD waveguide is formed by using high dielectric-constant materials for the purpose of achieving a smaller size, a phenomenon in which single mode transmission cannot be performed has been observed, as reported in Soube Shinohara et al., "Specific Transmission Characteristics of Nonradiative Dielectric Waveguide Using High Dielectric-Constant Materials", Journal of The Institute of Electronics, Information and Communication Engineers of Japan, C-I, Vol.J73-C-I, No.11, pp.716-723, November 1990). The reason why single mode transmission cannot be performed in the conventional NRD waveguide is that a very small gap between the dielectric strip and the metal plate of the NRD waveguide, which cannot be avoided in the manufacturing process, narrows the band of single mode transmission. In order to solve this problem, in Shinohara et al., a "trapped insular guide" (hereinafter referred to as a "third conventional example") has been proposed as a structural scheme for an arrangement using high dielectric-constant materials. However, this third conventional example has a problem in that the arrangement is complex, and the manufacturing steps are complex, resulting in a considerable increase in the manufacturing cost.